The reaction employing CuCN is well known in the art and is sometimes called the Rosenmund-von Braun Reaction. The reaction can schematically be represented by: EQU ArX+CuCn.fwdarw.ArCN+CuX
wherein X represents a halogen atom and the reaction is carried out at a temperature of about 150.degree. -250.degree. C. The reaction is carried out in a variety of solvents, the most commonly employed solvents being polar aprotic solvents such as dimethylformamide, dimethylacetamide and N-methyl-pyrrolidone. Commonly, an excess of CuCN is employed.
An example of such a process is the preparation of p-hydroxybenzonitrile (HBN) from p-halogenated phenol, which can conveniently be carried out in dimethylacetamide (DMA) as the solvent.
When the reaction is complete the solvent is normally distilled off and the nitrile product is obtained as a complex bound to the cuprous halide which is formed in the reaction. In order to isolate the nitrile product, the complex must be decomposed. Several methods are known in the art to effect this decomposition. In one such method (U.S. Pat. No. 3,259,646) concentrated hydrochloric acid and ferric chloride are added and the product is isolated by extraction into an organic solvent. According to another method [L. Freidman and H. Shechter, J.A.C.S., Vol. 26, pp. 2522-24 (1961)], ethylene diamine is added and the product is again isolated by solvent extraction. According to a further known method, the cuprous halide is dissolved in a aqueous solution containing 4 molar equivalents of NaCN and the product is extracted with an organic solvent.
The aforementioned and other known isolation methods present several serious drawbacks. The copper halide formed in the reaction is transformed into difficultly recoverable forms, which cannot be recycled as such. Furthermore, in the first mentioned method the formation of highly toxic hydrogen cyanide and cyanogen takes place. In the last two methods a considerable drawback is the alkaline nature of the solutions obtained, which prevents extraction of acidic nitriles into organic solvents.
In copending Israeli Patent Application No. 82694 of the same applicant, there is described a process for the preparation and the isolation of aromatic nitriles in which an aromatic halogenated compound is reacted with cuprous cyanide in a suitable solvent, comprising the steps of:
a) adding water to the reaction mixture obtained after the reaction of the aromatic halogenated compound with cuprous cyanide has been substantially completed; PA1 b) adding an alkali metal cyanide to the resulting mixture; PA1 c) separating the resulting CuCN suspension; and PA1 d) separating the organic phase to recover the aromatic nitrile; the amount of alkali metal cyanide added being substantially stoichiometric with the aromatic reagent employed in the reaction. PA1 a) contacting the reaction mixture obtained after the reaction of a p-halogenated phenol with cuprous cyanide in a suitable solvent has been substantially completed, with a mixture of an aqueous solution of a bromide salt and, optionally, with an organic diluent; PA1 b) separating the aqueous and organic phases; PA1 c) distilling HBN out from the organic phase; PA1 d) adding an alkali metal cyanide to the aqueous phase; and PA1 e) filtering CuCN off from the resulting aqueous phase;
The above-described process, while representing a considerable improvement over processes known in the art before, still suffers from the drawback that the recovery of the aromatic nitrile requires difficult filtration operations. Such filtration operations are both cumbersome, time-consuming and expensive.
Another drawback of the process of IL 82694 is that the recovered CuCN is contaminated with organic materials, and is therefore obtained with relatively low purity.